The annual report examines the server farms built by the largest Internet companies — including Amazon, Apple, Facebook, Google, Microsoft and Yahoo — and ranks them according to how efficient their cloud facilities are, and where they get their electricity.

Yahoo — which has struggled to please investors in recent years — was the only major Internet company in the study to get most of its electricity from renewable or clean energy sources, according to the report.

Many server farms can consume as much electricity as a small city. Some of the largest consume as much electricity as 180,000 homes.

Google and Yahoo historically have led the pack in trying to move more cloud computing toward renewable sources of energy. Now, they have some company in Facebook — which is building a server farm in Sweden that can be powered by renewable sources.

Amazon, Apple and Microsoft continue to rapidly add server capacity, but according to Greenpeace, don't seem to care where their electricity comes from.

All three rely heavily on coal, which Greenpeace calls a dirty and polluting source of power. On Greenpeace's report card, Amazon gets three F's and a D. Apple gets three D's and an F.

"When people around the world share their music or photos on the cloud, they want to know that the cloud is powered by clean, safe energy," said Gary Cook, Greenpeace International senior policy analyst. "Yet highly innovative and profitable companies like Apple, Amazon and Microsoft are building data centers powered by coal, and acting like their customers won't know or won't care. They're wrong."

According to Greenpeace's estimates, these three firms get roughly 85 percent of their power from coal on nuclear plants. The report doesn't break out natural gas as a source of power in the U.S.

These firms' servers use only about 15 percent or less of their power from renewable and other clean sources, according to the group. This puts them in the middle of the pack compared with other tech firms. Yahoo and Dell both get most of their power from renewable and clean sources.

However, some tech giants took issue with Greenpeace's numbers.

Greenpeace says it shared its findings with both Apple and Amazon. Both companies told the group the estimates were not accurate, but according to Greenpeace, they refused to provide more accurate information about their power usage.

Apple issued a statement in response to the report, arguing that Greenpeace grossly overestimated the energy usage at their data center in North Carolina and discounted efforts the company is making to use renewable energy.

"Our data center in North Carolina will draw about 20 megawatts at full capacity, and we are on track to supply more than 60% of that power on-site from renewable sources including a solar farm and fuel cell installation which will each be the largest of their kind in the country," said Kristin Huguet, an Apple spokesperson. "We believe this industry-leading project will make Maiden the greenest data center ever built, and it will be joined next year by our new facility in Oregon running on 100% renewable energy."

Amazon refused to discuss any details about energy consumption at their data centers, but called the report "inaccurate."

"Amazon Web Services believes that cloud computing is inherently more environmentally friendly than traditional computing," wrote Tera Randall, an Amazon spokesperson. "Instead of each company having their own datacenter that serves just them, AWS makes it possible for hundreds of thousands of companies to consolidate their datacenter use into a handful of datacenters in the AWS Cloud, resulting in much higher utilization rates and eliminating the waste that occurs when datacenters don't operate near their capacity.